The disclosure relates to a pressure-maintaining valve in the purge circuit of a closed hydraulic circuit.
Document DE 195 42 258 A1 shows a closed hydraulic circuit, via the two working lines of which a two-cylinder thick material pump (concrete pump) is supplied. The two working lines are alternately connectable via what is referred to as a purge valve to a purge line leading to the tank. In more precise terms, the respective working line conducting low pressure is connected to the tank via the purge valve. In order to ensure a minimum pressure in the purge line, a pressure-maintaining valve is provided therein.
Since the two cylinders of the thick material pump have to be supplied via the closed circuit in a differential mode, the variable displacement pump of the closed circuit is periodically fully pivoted. As a result, a corresponding periodic change between high pressure and low pressure arises in the two working lines. The pressure on the low pressure side may severely breakdown here and thus damage the variable displacement pump.
Document DE 10 2005 008 217 A1 likewise shows a closed circuit, the two working lines of which are alternately connectable via a purge valve to a purge line leading to the tank, wherein a pressure-maintaining valve is arranged in the purge line. Pressure drops in the respective working line conducting low pressure are intended to be prevented by the fact that, during the change-over operations of the variable displacement pump, the purge flow from the working line conducting low pressure through the purge line is briefly interrupted. In first exemplary embodiments, a directional control valve which is brought into a blocking position is provided for this purpose in series with the purge valve and with the pressure-maintaining valve. The comparatively large volumetric flow for which the additional directional control valve has to be designed is a disadvantage thereof. In a second exemplary embodiment, the control chambers on the purge valve are relieved via pilot valves to the tank, and therefore the purge valve passes into its central position and shuts off the purge flow.
According to document DE 10 2008 060 066 A1, a control chamber is provided on the spring side of the pressure-maintaining valve of the purge line, said control chamber being able to be acted upon with the pressure of an external pressure source in order thereby to shut the pressure-maintaining valve and to shut off the purge flow. The outlay in terms of device for the shut-off function of the pressure-maintaining valve is a disadvantage thereof.
By contrast, the disclosure is based on the object of providing a pressure-maintaining valve arrangement which can be switched off or can be shut off for a purge circuit of a closed hydraulic circuit in which the outlay in terms of device is reduced.